129 
have of course ejcplored 
energy but without 
has 
under much consideration is based on the fact that whereas a heavy nu- 
cleus gives out energy on breaking into two parts, i.e., when it under- 
goes fission, energy is liberated when two or more very light nuclei 
combine to form a slightly heavier nucleus. To be specific, we know 
that the helium nucleus is made up of two protons and two neutrons 
packed together and we know the exact value of its mass. We also know 
that the exact value of the mass of the helium nucleus is definitely 
smaller than the sum of the masses of the individual protons and neu- 
two Drotons and two 
together. So, if we form a helium nucleus 
mass, and consequently the evolved energy, are about seven times 
greater than when a U-235 nucleus undergoes fission. The only thing 
left is to find a way of putting the helium nucleus together from its foui 
component parts. This has not been done, and the outlook for doing it 
seems hopeless. 
Professor Bethe of Cornell proposed in 1939 a theory for the origin 
of the Sim's heat which is generally accepted. Deep down in the sun, 
the temperature is of the order of 20,000,000^0. This means that the 
atoms are moving about so rapidly that the electrons around the nuclei 
of different atoms now have some chance of colliding with each other. 
A collision between two nuclei provides an opportunity for a nuclear 
reaction. Two protons colliding may result in the creation of a deuter 
on and a positron. Energy is evolved. An alternative process which is 
more likely is a rather 
between orotons and carbon and 
In the 
^two protons and two neutrons disappear and a helium 
nucleus appears. The number of carbon atoms remains unchanged, they 
play the part of a catalyst. Since the mass of the product is less than 
the mass of the protons and neutrons one starts with, there is a loss of 
mass and therefore an evolution of energy. This theory gives results m 
excellent accord with the known facts about the sun's heat. The upshot 
of this is that we have very good reason for believing that the combina- 
tion of very light nuclei into slightly heavier ones evolves energy. This 
then gives us confidence in the view that somehow the synthesis of say 
a heUum nucleus from lighter nuclei on the earth would provide a great 
deal of energy. But what happens in the sun is no adequate guide to the 
reproduction of the process in a hydrogen bomb on the earth, for m the 
sun, time for the event to occur is unlimited, whereas in a hydrogen 
bomb it is essential that the whole process be over and done with m 
less than a millionth of a second so that the materials 
step? 
the available energy is set free. What 
can 
I suppose hundreds of physicists have doodled on paper witn sym- 
5 representing light nuclei and tried to combine them in all sorts c 
s to produce slightly heavier nuclei. Then by looking up tables oi 
exact masses of the nuclei concerned it is easy to calculate now 
:h mass is lost in the process and so to esUmate how much energy 
be set free. Perhaps the most obvious way of makmg a helium 
